Activation of a cathode

ABSTRACT

The present invention relates to a method for activation of a cathode comprising at least a cathode substrate wherein the cathode is cleaned by means of an acid, the cleaned cathode is coated with at least one electrocatalytic coating solution, drying the coated cathode until it is at least substantially dry, and thereafter contacting the cathode with a solvent redissolving precipitated electrocatalytic salts or acids formed on the cathode, originating from the electrocatalytic solution, to form dissolved electrocatalytic metal ions on the cathode surface, so that said electrocatalytic metal ions can precipitate as metals on the cathode. The invention also comprises a cathode obtainable by the method and the use of an activated cathode in an electrolytic cell for producing chlorine and alkali hydroxide.

[0001] The present invention relates to a method for activating acathode suitable for activation on site at a production plant. Theinvention also relates to the use of the activated cathode in anelectrolytic cell producing chlorine and alkali metal hydroxide.

BACKGROUND OF THE INVENTION

[0002] Electrodes are commonly, when in operation, immersed in anelectrolyte in an electrolytic cell where chemical products are producedby way of oxidation and reduction reactions of reactants present in theelectrolyte. The reduction reactions take place at the cathode wherereduction products are obtained. The oxidation reactions take place atthe anode where oxidation products are obtained.

[0003] Over time, the electrodes will become exhausted and deactivateddue to various deactivation processes taking place while theelectrolytic cells are in operation. In most electrolytic processes, theelectric energy is the most expensive “raw material” in the electrolyticreactions.

[0004] In the chlorine and alkali metal hydroxide production, it hasbeen found that the cathodes are liable to progressive deactivation overtime. The cathodes are subjected to deposition and precipitation ofmaterials present in the electrolyte and to other deterioratingprocesses deactivating the cathode. The decrease in activity leads to ahigher power consumption due to an increased overvoltage.

[0005] It is thus a big concern in electrolysis processes to provideactive cathodes throughout the whole electrolysis cycle.

[0006] Earlier attempts to solve this problem have involvedtransportation of the deactivated cathodes to the electrode manufacturerfor reactivation. However, the transportation of cathodes is a veryexpensive and time-consuming alternative to carry out. Another approachof providing active cathodes has involved replacement of the exhaustedcathodes with new ones.

[0007] U.S. Pat. No. 5,164,062 describes a method for preparing a newcathode comprising coating a cathode substrate of e.g. Ni with palladiumand another electrocatalytic metal. The pH of the coating solution maybe adjusted by an organic acid, e.g. acetic acid, oxalic acid and formicacid, or inorganic acids to maintain the pH below 2.8. However, theactivation by this method is not always satisfactorily increased.Furthermore, a portion of the active coating solution is wasted in themethod described above, because some of the acidic electrocatalyticcoating solution is rinsed away from the cathode substrate in order toavoid corrosion of the cathode. The rinsing solution that has taken upremaining electrocatalytic material must then be decontaminated fromsubstrate ions, e.g. nickel or other contaminating ions, which also arepresent on the cathode before the electrocatalytic material can bereused as coating material in an electrocatalytic solution again. Suchdecontamination procedure may involve several cleaning steps before theelectrocatalytic material has been satisfactorily cleaned.

[0008] The present invention intends to solve the above problems.

THE INVENTION

[0009] The present invention relates to a method of activating a cathodesuitable for the production of e.g. chlorine and alkali metal hydroxide.The term “activate” or “activation” etc. as used herein encompass bothactivation of a new electrode, which is to be prepared, and activationof an electrode, which has already been in operation in an electrolyticcell and which may have lost at least some of its initial activity.

[0010] It has been surprisingly found that the activation of a cathodecomprising at least a cathode substrate, which may have some remains ofan electrocatalytic coating on the substrate, can easily can beperformed on site at the production site. The method comprises at leastthe following steps:

[0011] cleaning the cathode by means of an acid

[0012] coating the cleaned cathode with at least one electrocatalyticcoating solution

[0013] drying the coated cathode until it is at least substantially dry,and thereafter contacting the cathode with a solvent redissolvingprecipitated electrocatalytic salts or acids formed on the cathode,originating from the electrocatalytic coating solution, to formdissolved electrocatalytic metal ions on the cathode surface, so thatthe electrocatalytic metal ions can precipitate as metals on thecathode.

[0014] The solvent must be able to redissolve any possible precipitatedelectrocatalytic salts or acids on the cathode originating from theelectrocatalytic coating solution. The solvent may contain a smallamount of electrocatalytic metals dissolved therein, which may originatefrom a rinsing solution containing remains of an electrocatalyticsolution. The contacting of the solvent with the cathode is suitablyperformed by spraying or in any other way putting solvent on the cathodein a suitable amount.

[0015] By the term 'substantially dry” is meant a coated cathode whichcontains only a small quantity of solution on its surface such that thesolution does not substantially flow away from the cathode. Suitably,such quantity ranges from about 0 to about 10 ml/m², preferably fromabout 0 to about 5 ml/m² solution.

[0016] The cathode comprises a substrate of e.g. nickel, cobalt, copper,iron, steel, particularly stainless steel, or alloys or mixturesthereof, preferably nickel. The cathode may also comprise remains of anelectrocatalytic coating deposited on the substrate, and/or contaminantsfrom an electrolytical process.

[0017] Used cathodes are preferably disassembled from the cells beforeactivation.

[0018] According to one embodiment, the cathode is welded to a pan. Theused cathode pan structure, i.e. the cathode and the pan, is preferablydisassembled and removed from the cell before activation. Forsimplicity, the term “cathode”, where otherwise not stated, willhenceforth also signify the cathode pan structure.

[0019] The cathode is cleaned with a cleaning solution comprising atleast one acid. The pH of the cleaning solution is suitably adjusted byaddition of an inorganic acid, e.g. HCl, H₂SO₄, HNO₃, or an organicacid, e.g. oxalic acid or other organic acids, or mixtures thereof,suitably to a pH from about −1 to about 6, preferably from about −1 toabout 3. The acid reacts with the cathode substrate and is also believedto react with precipitated substances on the substrate and theelectrocatalytic coating. The cleaning time is not critical and mayrange from about a few minutes to about 30 minutes or more. Thetemperature during the cleaning is not critical and may be at e.g. roomtemperature, suitably the temperature ranges from about 0 to about 100°C., preferably from about 0 to about 35° C.

[0020] According to one preferred embodiment of the invention, also areducing agent is comprised in the cleaning solution which is believedto prevent corrosion of the cathode and facilitate removal ofdeactivating precipitates on remaining electrocatalytic coating. Thereducing agent is also believed to stabilise activated areas of thecathode. The reducing agent may be present in the cleaning solution at aconcentration of from about 0.5 to about 50 wt %, preferably from about0.5 to about 10 wt %. The reducing agent is suitably selected fromalcohols such as isopropyl alcohol or n-pentanol, HCl, H₃PO₂, H₃PO₃,N₂H₄, NH₂OH, NH₃, Na₂S, NaBH₄, sodium hypophosphite (NaH₂PO₂),dimethylamine borane (CH₃)₂NHBH₃, or mixtures thereof. Preferredreducing agents are selected from HCl, H₃PO₂, H₃PO₃, N₂H₄, NH₂OH, andNH₃, and most preferably from HCl.

[0021] After the cleaning; the cathode is suitably rinsed and dried. Thecathode is then contacted with at least one electrocatalytic coatingsolution, comprising an electrocatalytic metal and preferably acomplexing agent.

[0022] According to one embodiment of the invention, severalelectrocatalytic coating solutions, e.g. two or more coating solutions,may be contacted with the cathode. The coating solutions are suitablycontacted with the cathode one after the other, preferably when thepreviously applied coating solution has dried on the surface of thecathode.

[0023] The electrocatalytic coating solution or solutions are suitablyapplied by means of painting, rolling or any other plausible methodsuitable for on-site coating. The electrocatalytic coating solutionsuitably comprises one or several noble metals in the form of salts oracids or the like, selected from the platinum group, e.g. Ru, Rh, Os,Ir, Pd, Pt, Au, Ag, or alloys or mixtures thereof. The noble metals cansuitably be present in the coating solution at a concentration fromabout 25 to about 200, preferably from about 50 to about 150 gmetal/litre coating solution. The electrocatalytic metals are suitablyderived from salts or acids of e.g. platinum metals such as hexa chloroplatinum acid, platinum metal alcoxi complexing materials, chlorides orthe like. The coating time of the cathode is not critical and may be forabout one hour or more. The temperature of the coating solution suitablyis room temperature, but may range from about 0 to about 100° C. Thecoating procedure is suitably carried out within the same temperaturerange, i.e. 0-100° C., preferably between 0 and 35° C. Also a complexingagent may be added to the coating solution preferably in a concentrationof from about 100 to about 500, and most preferably from about 350 toabout 450 g/litre coating solution. The optionally added complexingagent facilitates the oxidation and reduction reactions taking placewhen the coating solution is contacted with the substrate. The substratemetal of the cathode is spontaneously oxidised to its correspondingionic form whereas the electrocatalytic metal or metals in the coatingsolution is reduced from its ionic form to its metallic form therebyforming an electrocatalytic coating on the substrate. It has been foundthat the complexing agent supports the reduction/oxidation reactiontaking place so as to improve the precipitation reaction and theadherence of the electrocatalytic metal to the substrate. Suitablecomplexing agents comprise hypophosphorous acid, sulphurous acid,nitrous acid, alcohols such as glycol, glycerine, acetate, propionate,succinate, hydroxyacetate, α-hydroxypropionate, aminoacetate,ethylenediamine, β-aminopropionate, malonate, pyrophosphate, malate,citrate, ammonium salts, EDTA, or mixtures thereof.

[0024] The coated cathode is then allowed to dry so it becomes at leastsubstantially dried, suitably from about 0 to about 10 ml/M², preferablyfrom about 0 to about 5 ml/m². Preferably, the coated cathode iscompletely dried before it is contacted with the solvent. The driedcathode is then contacted with a solvent suitably comprising a reducingagent. It has been surprisingly found that the contacting of the solventwith the cathode results in a lower overpotential, often 10-30 mV loweror more, than a cathode not treated in this manner. The solvent maycomprise water, suitably in combination with HCl, H₃PO₂, H₃PO₃, H₂O₂,N₂H₄, NH₂OH, NH₃, Na₂S, Na₂SO₃,K₂SO₃, a alcohol, n-pentanol, or mixturesthereof. The lower overpotential is considered to be principally due toa higher deposit level of electrocatalytic metals on the activatedcathode. The concentration of a possible reducing agent in the solventsuitably ranges from about 10 to about 70 wt %, preferably from about 40to about 50 wt %. The temperature during the contacting of the cathodewith a solvent suitably ranges from about 8 to about 60° C., preferablyfrom about 15 to about 35° C. The reaction time during whichelectrocatalytic metals can precipitate as metals on the cathodesuitably is from about 1 to about 60 minutes or until the electrode iscompletely dried. Suitably, solvent can thereafter again be deposited onthe cathode to repeat the precipitation procedure of electrocatalyticmetals in case such remains exist on the cathode in the form of salts oracids. Suitably, an amount from about 10 to about 100 ml solvent/m²cathode area is contacted with the cathode, preferably from about 50 toabout 100 ml solvent/m².

[0025] The activated cathode is then preferably rinsed by a rinsingsolution such as water to avoid corrosion, preferably with a basicsolution such as NaOH after that the solvent on the cathode hassubstantially dried. Preferably, the basic rinsing solution has aconcentration of e.g. NaOH from about 0.0001 to about 50 wt %, and mostpreferably from about 0.0001 to about 20 wt %.

[0026] The activated cathodes are usually run in the electrolytic cellsuntil their activity is found to be too low, i.e. at an uneconomicallylow level. This crucial extent of deactivation can be optimised by aperson skilled in the art by estimating the electric energy consumed andthe activation costs. When the reactivation is to be initiated, the usedcathodes are preferably disassembled and removed from their cells.Suitably, the reactivation can be performed in connection withreplacement of the membranes arranged in the electrolytic cell.

[0027] The present invention also relates to a cathode obtainable by themethod as described above.

[0028] The invention further concerns the use of an activated cathode inan electrolytic cell for producing chlorine and alkali metal hydroxide.

[0029] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the gist and scope of the present invention, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the claims. The followingexamples will further illustrate how the described invention may beperformed without limiting the scope of it.

EXAMPLE 1

[0030] A cleaning solution was prepared from concentrated hydrochloricacid (37 wt %) to yield a final concentration of 20 wt % hydrochloricacid. The cathode to be activated was contacted with the cleaningsolution by means of painting. 50 ml cleaning solution/m² geometriccathode area was applied. The solution was then allowed to react during10 minutes at room temperature (25° C.). The cathode was thereafterrinsed thoroughly with deionised water. Meanwhile, a coating solution ofRhCl₃ was prepared by dissolving the rhodium salt in a 20 wt %hydrochloric acid solution, resulting in a final rhodium concentrationof 50 g Rh metal/litre coating solution. Also a coating solution ofRuCl₃ was prepared by dissolving the Ru salt in another 20 wt %hydrochloric acid solution resulting in a concentration of 50 g Rumetal/litre coating solution. The rinsed cathode was allowed to dry inroom temperature, whereafter the Rh coating solution was applied theretoin an amount of 50 ml/m² geometric cathode area by means of painting.The cathode was then allowed to dry for 1 hour. The Ru coating solutionwas then applied to the Rh coated cathode in an amount of 50 ml/M²geometric cathode area. The cathode was then dried whereafter an aqueoussolution of 50 wt % H₃PO₂ was painted on the cathode. Thereafter, thecathode was allowed to dry whereupon it was rinsed with water. Thecathode obtained showed satisfactory activation.

EXAMPLE 2

[0031] Two deactivated nickel-based cathode samples P1 and P2 werecleaned by means of painting with a 20 wt % hydrochloric acid solutionfor 5 minutes. The cathode samples were thereafter rinsed with water andthereafter dried. The two samples were both coated with a 40 ml RhCl₃coating solution having a rhodium content of 150 g/litre/m². The coatedsamples were then allowed to dry for 1 hour. Unreacted rhodiumprecipitated during the drying stage and formed rhodium chloride salt onthe cathode substrate. The P1 sample was gently rinsed with a causticsolution having a pH of 10, whereupon unreacted precipitated rhodiummetal salt (RhCl₃) and nickel chloride were rinsed off the cathodesample. The remaining amount of rhodium on the P1 sample only amountedto a small portion of the initially precipitated rhodium content. Thiswas judged from the rhodium colour the rinsing solution got since themetal was partially washed off. The P2 cathode sample was gently sprayedwith a 20 wt % hydrochloric acid after the RhCl₃ solution had dried onthe sample, whereupon precipitated RhCl₃ was redissolved. Subsequentprecipitation of metallic rhodium could then take place on the P2sample. The addition of hydrochloric acid to the P2 cathode was repeatedonce after that the P2 cathode had dried. 15 minutes after the secondaddition of hydrochloric acid, i.e. after that the cathode wassubstantially dry, the cathode was rinsed with caustic solution in thesame manner as the P1 sample. No colour shift could be observed in therinsing solution due to rinsed off rhodium. It was thus shown that amuch higher amount of rhodium had adhered to the P2 sample than to theP1 sample as a result of adding the solvent to the coated and driedsample. Electrolytic trials performed involving use of the activatedcathodes showed that the cell voltage was 230 mV lower for the P2cathode than for the P1 cathode when the used electrolytic cell wasoperated at a current density of 4.7 kA/m².

1. Method for activation of a cathode comprising at least a cathodesubstrate characterised in that the cathode is cleaned by means of anacid the cleaned cathode is coated with at least one electrocatalyticcoating solution drying the coated cathode until it is at leastsubstantially dry, and thereafter contacting the cathode with a solventredissolving precipitated electrocatalytic salts or acids formed on thecathode, originating from the electrocatalytic solution, to formdissolved electrocatalytic metal ions on the cathode surface, so thatsaid electrocatalytic metal ions can precipitate as metals on thecathode.
 2. A method according to claim 1, wherein the cathode substrateis selected from nickel, cobalt, copper, iron, steel, or alloys ormixtures thereof.
 3. A method as claimed in any of the preceding claims,wherein the cathode substrate is nickel.
 4. A method as claimed in anyof the preceding claims, wherein the electrocatalytic coating solutioncomprises a complexing agent.
 5. A method according to claim 4, whereinthe complexing agent is selected from at least one of hypophosphorousacid, sulphurous acid, nitrous acid, alcohols, glycerine, acetate,propionate, succinate, hydroxyacetate, α-hydroxypropionate,aminoacetate, ethylenediamine, β-aminopropionate, malonate,pyrophosphate, malate, citrate, ammonium salts, EDTA, or mixturesthereof.
 6. A method as claimed in any of the preceding claims, whereinthe solvent comprises water.
 7. A method as claimed in any of thepreceding claims, wherein the electrocatalytic coating solution containssalts or acids of Pt, Rh, Ru, Pd, Ir, Os, Ag, Au or alloys or mixturesthereof.
 8. A method as claimed in any of the preceding claims, whereinthe activation is performed on a used cathode.
 9. A method as claimed inany of the preceding claims, wherein the activated cathode is rinsedwith a basic solution.
 10. A cathode obtainable by the method accordingto any of the preceding claims having a lower overpotential, often 10-30mV lower or more, than a cathode not treated according to said method.11. Use of an activated cathode according to claim 10 in an electrolyticcell producing chlorine and alkali metal hydroxide.